okay, first thing to get straight, you can NOT get pregnant from semen, you get pregnant from SPERM. semen is not the same as sperm. precum is semen, but it CAN contain sperm. when precum is released it flushes out any sperm that is left inside, so there CAN be sperm in precum. when a man ejaculates, he is releasing a mixture of semen and sperm.
but sperm can only fertilize an egg that it has access to. the digestive system and the ovaries are not connected at all, so there is absolutely no way swallowing alone can get you pregnant. there is a very small chance of getting pregnant if you get sperm in your mouth, kiss him (transferring it to his mouth) and then he gives you oral sex. unless you transfer it to your vagina somehow, you cant get pregnant from it.